In 2.50, you can do it in your own admin template without the need for a plugin.
1) Copy one of the templates from
/styles to
/upload/admintheme/ and rename it (let's say "yourTheme").
2) Add a custom.js file to
/upload/admintheme/yourTheme/scripts/
3) Modify
/upload/admintheme/yourTheme/config.xml as follows:
- Change the name, description and any other <metadatas> you see fit
- Modify the <js> files to include your new custom.js
Code:
<js>
<filename>notify.js</filename>
<filename>save.js</filename>
<filename>panelclickable.js</filename>
<filename>panelsanimation.js</filename>
<filename>sidemenu.js</filename>
<filename>bootstrap-switch.min.js</filename>
<filename>jquery.jcarousel.min.js</filename>
<filename>jcarousel.responsive.js</filename>
<filename>custom.js</filename>
</js>
4) Switch Global settings --> General --> Administration template to "yourTheme"